Field of the Invention
The invention pertains to apparatus and methods for densifying thermoplastic polymers, and more particularly to methods for creating dense thermoplastic films with improved crystallinity on selected substrates.
Description of Related Art
Polyimides are attractive materials for the microelectronics industry because of their excellent mechanical, electrical, and chemical properties. The process time for conventional thermal curing typically ranges from 4 to 6 hours; slow temperature ramp rates and extended hold times at various temperatures are needed to allow for slow reaction rates, outgassing of reaction by-products and solvent, and orientation of polymer chains. Reducing the processing time required to cure these polymers would increase throughput and reduce overall production costs.
Polyamic acid based polyimides such as 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (BPDA) with p-phenylenediamine (PPD) are desirable for electronic packaging applications where a low residual stress dielectric is essential. Many of the unique properties of this polymer are attributed to the rigid nature of its backbone and the high degree of orientation that occurs during cure. This orientation is critical to achieving the low coefficient of thermal expansion (CTE), creating a low stress film.
One obvious shortcoming of this type of polymer system is the high cure temperature (typically 350° C.), which precludes its use in many advanced semiconductor systems, where the small feature size and correspondingly reduced diffusion distances severely limit the thermal budget available for the various process steps. For instance, a recent paper reported desirable properties of this polymer system, Table 1, but all of the films reported had been processed at 310-350° C., whether by conventional oven curing, rapid hotplate curing, or microwave curing [K. D. Farnsworth et al., Variable Frequency Microwave Curing of 3,3′,4,4′-Biphenyltetracarboxylic acid dianhydride/P-Phenylenediamine (BPDA/PPD), Intl. Journal of Microcircuits and Electronic Packaging 23:162-71 (2002)]. Although the VFM cure was significantly faster, cure temperature was unchanged, and cure temperatures in this range are well beyond the allowable maximum temperature for many applications of interest. The difficulty in applying these polymer systems to demanding electronic applications can be seen by the fact that the commercial version of BPDA/PPD has been available for more than thirty years, and yet had very limited use.
TABLE 1Typical Cured PI 2611 Properties by Prior Art MethodsMethodPropertyThermal CureHotplateVFMFinal cure temperature, ° C.350350350Total cure time, s18,00036001200Residual stress, MPa635.34.2on [100] SiCTE, ppm/° C.36.63.8Tensile strength, GPa>0.374>0.0841>0.361Dielectric constant at 10 kHz3.063.093.34Loss tangent at 10 kHz0.00320.004260.0033Birefringence0.2249—0.2237Degree of cure100%87-108%82-102%Degradation temperature, ° C.539—540